Balancing systems for single cylinder engines have been known a long time. If no balancing at all is applied inertia forces corresponding to the full weight of the piston including piston pin and a part of the crank rod will act up and down the longitudinal direction of the cylinder. By arranging the crank webs to provide an unbalance force of half the above weight in a counter direction, for short called half piston weight, the maximum unbalance force will be reduced to half but acting in all directions. This arrangement provides the lowest unbalance forces. If instead the crank webs provide an unbalance force equal to the full weight of the piston the result would be similar to no balancing, but the unbalance forces would in this case act only in a perpendicular direction to the direction of the cylinder and be of full size, i.e. corresponding to the weight of the piston. Therefore the full piston weight balancing will provide twice as high forces as the half piston weight balancing, but acting in only one direction.
Portable, hand-held power cutters powered by a single cylinder internal combustion engine are known since long. These machines are often used for cutting concrete and similar materials. Such a cutting creates a lot of abrasive particulate matter. Typically, efficient air cleaning is vital and is attained mainly through a filter assembly having a big filter volume, which may increase a service life of the machine. To create a big filter volume in a compact machine, power cutters have been introduced which have an engine cylinder which is leaned forwards. An example is Husqvarna K 750 and K760. This design is described in U.S. Pat. No. 7,520,276, assigned to Husqvarna AB and the disclosure of which is hereby incorporated by reference in this application.
However this kind of design layout has a forward leaning cylinder with a tilt angle (α) that is between 5 degrees and 40 degrees in contrast to a more traditional design with a backwards leaning cylinder, i.e. with a negative tilt angle. Further the cutter arm points more upwards than in a traditional design. These differences contributes in creating an acute angle between the cutter arm centerline and the cylinder bore centerline, and this acute angle is larger than 45° but smaller than 75°. It is typically 55°-60°, compared to 80°-100° for a more traditional design. A result of all these differences is that the unbalance forces and torques created by the single cylinder engine gives a very different, more complex and more severe strain situation for the cutter arm. Especially this is true for an engine speed corresponding to a resonance frequency for bending the cutter arm.
U.S. Pat. No. 4,836,297 and its German counterpart DE 3 546 029 describe a balancing system for reducing handle vibrations of a power tool having a cylinder with a strong backwards tilt. Further its crankshaft has a counterbalancing structure with crank webs for counterbalancing the full weight of the piston. This creates high unbalance forces from the crank webs. These would need to be compensated by further big unbalances. All this creates extra stress in the structure, e.g. in a cutter arm. Also it is only possible to balance in a direction perpendicular to the cylinder axis, as the unbalance forces act in this direction. As the cutter arm normally does not run in this direction, such a balancing system can not minimize bending stress in the cutter arm, and the purpose was to reduce handle vibrations. Further this type of balancing creates extra weight in the crank webs and extra weight in a flywheel or clutch for compensating the extra weight in the crank webs. This is a disadvantage for a hand held machine.
U.S. Ser. No. 08/865,934 describes a balancing system for primarily reducing vibrations in the handles caused by reaction forces from the tool in a hand held machine also having a cylinder with a backwards tilt. This reduction of vibrations are also said to reduce stress in a hand held machine. Also this design uses a counterbalancing structure with crank webs having weights 20 for counterbalancing the full weight of the piston. This is evidenced by the description and FIG. 2 “the balance weights 20 compensate the forces from the piston which are directed in the direction of movement of the piston”. Therefore it is only possible to balance in a direction perpendicular to the cylinder axis, as the unbalance forces act in this direction. As the cutter arm normally does not run in this direction, such a balancing system can not minimize bending stress in the cutter arm, but could of course reduce vibrations in the handles. And when testing a balancing as described it did not reduce stress in the cutter arm of a K750 or K760 power cutter, but only possibly to reduce maximum handle vibrations a little. So the recommendations of this document did not and can not solve the problem of reducing cutter arm fatigue, in fact it teaches away from a solution.
In light of the foregoing, there is a need for a solution reducing stress in the cutter arm of the above mentioned type of power cutters, which solution may have a simple design and a low weight or low weight increase.